The present invention is related to the field of low dielectric constant materials prepared by chemical vapor deposition (CVD) methods which serve as insulating layers in electronic devices. In particular, the present invention is directed to compositions for use as precursors to the low dielectric constant materials that have predetermined concentration limitations of certain impurities to eliminate process problems related to precipitation of such impurities.
The electronics industry utilizes dielectric materials as insulating layers between circuits and components of integrated circuits (IC) and associated electronic devices. Line dimensions must be reduced in order to increase the speed and memory storage capability of microelectronic devices (e.g., computer chips). As the line dimensions decrease, the insulating requirements for the interlayer dielectric (ILD) become more rigorous. Shrinking dimensions requires a lower dielectric constant to minimize the RC time constant, where R is the resistance of the conductive line and C is the capacitance of the insulating dielectric layer. C is inversely proportional to spacing and proportional to the dielectric constant (k) of the ILD.
Conventional silica (SiO2) CVD dielectric films produced from SiH4 or TEOS (tetraethylorthosilicate) and oxygen have a dielectric constant (k) of greater than 4.0. There are several ways in which the industry has attempted to produce silica-based CVD films with lower dielectric constants, the most successful being the doping of the insulating film with carbon atoms, fluorine atoms, or organic groups containing carbon and fluorine. Doping the silica with carbon atoms or organic groups lowers the k of the resulting dielectric film for several reasons. Organic groups, such as methyl, are hydrophobic; thus, adding methyl or other organic groups to the composition can act to protect the resulting CVD deposited film from contamination with moisture. The incorporation of such organic groups also serves to “open up” the structure of the silica, possibly leading to lower density through space-filling with bulky CHx bonds. Organic groups are also useful because some functionalities can be incorporated into the organosilicate glass (OSG), then subsequently “burned out” or oxidized to produce a more porous material which will inherently have a lower dielectric constant.
Carbon can be incorporated into an ILD by using an organosilane as the silicon source material in the PECVD reaction. An example of such would be the use of methylsilanes, (CH3)xSiH(4-x), as disclosed in U.S. Pat. No. 6,054,379. Alkoxysilanes (also referred to herein as silyl ethers) have also been disclosed as effective precursors for the introduction of organic moieties into the ILD. Particularly useful alkoxysilanes are disclosed in U.S. Pat. No. 6,583,048. Of such alkoxysilanes, diethoxymethylsilane (DEMS) has found significant commercial use.
The manufacture of organosilanes or alkoxysilanes, however, typically requires the use of halosilane chemical staring materials such as, for example, chlorosilane or organochlorosilane. In such reactions, the alkoxy group replaces the halogen, forming the desired alkoxysilane. Dimethyldimethoxysilane (DMDMOS), for example, is commercially manufactured utilizing the chemical reaction of dimethyldichlorosilane with methanol as shown below:(CH3)2SiCl2+2CH3OH→(CH3)2Si(OCH3)2+2HCl  (i)
In a similar manner, DEMS is typically prepared primarily by one of two synthetic routes: the “direct” synthesis, shown below by equation (ii), involving the reaction of dichloromethylsilane with ethanol; and the “orthoformate” synthesis, shown by equation (iii), which involves the reaction of dichloromethylsilane with triethylorthoformate:CH3SiCl2H+2CH3CH2OH→CH3Si(OCH3)2H+2HCl  (ii)CH3SiCl2H+2(CH3CH2O)3CH→CH3Si(OCH3)2H+CH3CH2Cl+2CH3CH2OC(O)H  iii)
In all of the above cases the synthesis of the desired alkoxysilane is accompanied by the production of stoichiometric quantities of chloride-containing byproducts such as hydrochloric acid (HCl), as in the case of the reactions (i) and (ii), or ethylchloride, (CH3CH2Cl) as in the case of the latter reaction. The crude product mixture also typically contains some amount of unconverted chloromethylsilane. This is particularly true for the synthesis of DEMS, in which it is not practical to treat the dichloromethylsilane starting material with a substantial molar excess of reactant in order to drive the reaction to quantitative conversion. The presence of Si—H in the dichloromethylsilane makes it particularly vulnerable to attack forming undesirable side-reaction products if exposed to a substantial excess of either ethanol (CH3CH2OH) or triethylorthoformate ((CH3CH2O)3CH). Given these constraints, the crude DEMS product typically has a significant amount of acid chlorides (HCl) and/or complexed silicon chloride impurities. Distillation is effective for removing most of the chloride impurities, but has limited efficacy for reducing the chlorides to the low levels required for CVD precursor source chemicals (e.g., <10 ppm by weight). In order to achieve these low chloride levels the product can be treated with a basic scavenger which will remove the chloride through complexation or adsorption. The basic scavenger can be in the form of a pure liquid or solid, such as in the case of an organoamine, in the form of a resin material such as in a packed bed of solid adsorbent material, or in the form of a gas such as, for example, gaseous ammonia.
Industry standards for ILD source materials have stringent requirements for low levels of residual chloride and nitrogen-containing components. Residual chloride presents integration issues due to its potential migration and high reactivity. Nitrogen also needs to be minimized because of its potential for diffusion, possibly causing resist poisoning issues. Consequently, unacceptably high levels of halogen or nitrogen in CVD feed materials may cause undesirable performance problems for the resulting ILD films.
As described above, two common routes for the preparation of DEMS are exemplified, each of which may yield unacceptably high levels of chloride contamination in the crude product due to unreacted starting material, acid chloride or complexed chloride byproducts. Distillation is commonly employed to purify the crude product, but it is not an effective means for reducing the chloride to acceptable levels. Typically, the distilled DEMS product is treated with an appropriate amount of a halide scavenger material in order to complex the chloride as the corresponding insoluble salt. This halide scavenger is often basic in nature, examples of which include amines, imides, alkali metal alcoholates, metal alkoxides, or solid adsorbent or resin materials such as activated carbons, alkali-treated activated carbons or other base-treated adsorbent substrates. The scavenger-chloride salt, thus formed, can be separated by conventional means such as filtration or further distillation in order to produce a DEMS product with less than 10 ppm chloride by weight.
There are significant drawbacks, however, associated with the use of residual chloride scavengers. For example, during CVD processing it is not uncommon that different lots of organosilicon precursor such as, for example, DEMS, may be combined, such as when a partially empty container is back-filled with a second source container of precursor, or when two different precursor source containers are feeding a common manifold. Precipitation of solids may occur if a sample of precursor containing a substantial amount of dissolved residual chloride is combined with a second source of precursor containing a substantial amount of dissolved residual basic scavenger. Solids formation in this manner leads to production problems because the solid precipitate typically restricts or blocks the flow of the liquid precursor, contaminates the liquid delivery or deposition hardware, and numerous potential performance and or quality issues associated with the deposited low-k films. Accordingly, there is a need in the art for an organosilicon precursor composition that is substantially free of having the potential to precipitate chloride salts upon mixture with other organosilicon precursor material.